fix routing issue for esoteric characters in gorilla/mux (#8967)

First step is to ensure that Path component is not decoded
by gorilla/mux to avoid routing issues while handling
certain characters while uploading through PutObject()

Delay the decoding and use PathUnescape() to escape
the `object` path component.

Thanks to @buengese and @ncw for neat test cases for us
to test with.

Fixes #8950
Fixes #8647

cmd/object-api-utils.go

@@ -165,10 +165,6 @@ func IsValidObjectPrefix(object string) bool {
 	if !utf8.ValidString(object) {
 		return false
 	}
-	// Reject unsupported characters in object name.
-	if strings.ContainsAny(object, `\`) {
-		return false
-	}
 	if strings.Contains(object, `//`) {
 		return false
 	}